This application requests funds to support a study of the specificity and mechanism of action of a variety of Na+ and Ca++-channel antagonists in a new preparation: the single bullfrog atrial cell. Within the past two years, my colleagues and I have perfected a method of enzymatically isolating viable single artrial cells from the bullfrog heart and a method of voltage-clamping these cells which has revealed the consistent presence of iNa, iCa and iK. In comparison to multicellular cardiac preparations, this preparation offers a number of important advantages for quantitative voltage-clamp and pharmacological studies: (i) the ability to directly study the influence of pharmacological agents on the fast inward sodium current and Vmax of the action potential in the same preparation and, (ii) the ability to carefully access the specificity of pharmacological agents on each component ionic current under conditions in which contamination by other currents is minimal. The mechanism of block of iNa by tetrodotoxin and two prototype antiarrhythmic agents, quinidine and lidocaine, will be studied. These experiments will carefully assess the relative importance of toxic verses voltage and/or use-dependent block of iNa by these agents. Additional effects of quinidine and lidocaine on steady-state currents and iK will be carefully examined. These data will clarify the mechanism(s) by which these agents modify action potential duration, refractory period, and membrane responsiveness in terms of changes in individual ionic currents. In additional experiments, the mechanism of iCa block by a series of polyvalent cations and organic Ca-channel antagonists will be studied. The specificity of each of these agents will also be examined in terms of additional modifications of iNa, iK and steady-state currents. The proposed experiments will provide important new information regarding the specificity and mechanism of antiarrhythmic action of these agents at the cellular level. Such information will contribute to a better understanding of cardiac arrhythmias in general and may lead to the development of newer and more effective antiarrhythmic agents.